In many of injection molding apparatuses, a mold is closed by moving in the same direction as an axial direction of an injection unit. In contrast to such injection molding apparatuses, some injection molding apparatus has a mold arranged to be closed by moving in a direction perpendicular to an axis of an injection unit, as disclosed in JP-U-46-26300.
FIGS. 15A and 15B hereof schematically show a well-known injection molding apparatus 200 disclosed in JP-U-46-26300. As shown in FIG. 15A, the injection molding apparatus 200 includes a base 201, an injection unit 202 disposed horizontally on the base 201, and a mold clamping mechanism 203 vertically movably disposed on the base 201.
In the mold clamping mechanism 203, a mold assembly 207 is shown in a closed position where a first cavity 208 is formed between an upper mold 204 and an middle mold 205 and a second cavity 209 is formed between the middle mold 205 and a lower mold 206.
Referring to FIG. 15A, the injection unit 202 injects a molten resin into the first cavity 208 after which a high-pressure oil is fed to a hydraulic cylinder 211 to elevate the mold clamping mechanism 203 in its entirety.
When the mold clamping mechanism 203 is elevated to a predetermined height, the injection unit 202 injects a resin material into the second cavity 209, as shown in FIG. 15B.
Since two molded products can be obtained using the single injection unit 202, productivity of the injection molding apparatus 200 is improved.
The mold assembly 207 gradually expands due to heat of the molten resin. This expansion of the mold assembly 207 changes heights of first and second sprues 212, 213. This results in each of central axes of the first sprues 212, 213 being vertically displaced out of alignment with a central axis of the injection unit 202 during injection of molten resin. If this vertical displacement is large, undesirable leakage of molten resin is likely to occur.
As for the injection molding apparatus 200 having the mold arranged to be closed by moving in the direction perpendicular to an axis of the injection unit 202, the injection unit 202 disposed on the base 201 does not move vertically whereas the sprues 212, 213 are vertically displaced in a direction parallel to a direction in which the mold moves when the mold is closed. As a result, leakage of molten resin is likely to occur around the sprues. Measures should be taken to address the leakage of the molten resin.